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Astron. Astrophys. 346, 922-928 (1999)

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1. Introduction

Magnetic fields play an essential role in stellar angular momentum evolution because the only way to extract angular momentum from a single star is magnetic coupling to the surrounding gas. This mechanism is particularly important in classical T Tauri systems (CTTS), where the stellar magnetic fields may thread the surrounding protoplanetary accretion disks.

In this type of system, the magnetic field prevents disk accretion inside the corotation radius and forces the gas to flow along the field lines towards the poles of the star (Camenzind 1990; Königl 1991; Shu et al. 1994). Outside the corotation radius, the disk is threaded by the field which exerts a positive torque on the disk, i.e. transfers angular momentum from the star to the disk and regulates the stellar rotation (Bouvier et al. 1993; Cameron & Campbell 1993; Edwards et al. 1993; Yi 1994; Ghosh 1995; Armitage & Clarke 1996; Li et al. 1996).

In the papers listed above, a dipolar magnetic field has been assumed with the axis of the dipole aligned with the axis of rotation. As long as the true field geometry is unknown, this assumption is the most reasonable one. As the efficiency of the disk braking mechanism strongly depends on the structure and strength of the stellar magnetic field (Wang 1995), this assumption must, however, be checked. Spot distributions obtained by Doppler imaging show non-axisymmetric patterns with large polar spots (Joncour et al. 1994a, 1994b; Hatzes 1995; Rice & Strassmeier 1996; Johns-Krull & Hatzes 1997) and do not support the assumption of an aligned dipole.

While the magnetic field of a CTTS is influenced by the presence of the disk, that of a weak-line T Tauri star (WTTS) is completely determined by the properties of the star. As T Tauri stars are fully convective (at least during the early T Tauri phase), a fossil field can be excluded. It would not survive longer than a few hundred years because the convective motions enhance the efficiency of Ohmic dissipation for large-scale fields by about ten orders of magnitude. The field must be the result of a dynamo, which, due to the lack of both differential rotation and a radiative core, must be expected to be quite different from the solar dynamo (Küker & Rüdiger 1997).

The most likely process is an [FORMULA]-dynamo, which is known to produce non-axisymmetric fields under certain conditions (Rüdiger & Elstner 1994). Such a dynamo has been proposed by Schüssler (1975) as a mechanism to produce the magnetic fields of Ap stars, which are radiative on the main sequence but pass a fully convective phase during their pre-main sequence evolution. As his model is axisymmetric, has a prescribed symmetry with respect to the equatorial plane, and contains an isotropic [FORMULA]-effect, its predictions about the field geometry are rather limited. A case much more similar to the one we deal with is that of Moss & Brandenburg (1995, MB), who treated a planetary dynamo in three dimensions with an anisotropic [FORMULA]-effect and indeed found non-axisymmetric geometry.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999